Figure
2.25The cladoceran Penilia avirostris, which is probably now established in Danish waters. Photo: Per Juel Hansen.

Zooplankton

The zooplankton are an important link between the phytoplankton and higher
trophic levels such as fish. Zooplankton constitute a diverse group of
organisms with respect to size, life cycle and behaviour. The smallest
size class, nanozooplankton (2-20 µm), are single celled organisms.
The larger size class, the microzooplankton (20-200 µm) are dominated
by ciliates and heterotrophic dinoflagellates, small metazooans as rotiferans
and the earliest developmental stages of copepods. The largest size class,
mesozooplankton (> 200 µm), consist of copepods, cladocerans
and larvae of benthic invertebrates. The more conspicuous jellyfish often
dominates the gelatinous plankton.

Due to the doubtful usefulness of zooplankton as an indicator of eutrophication,
zooplankton have been given relatively low priority in the Danish monitoring
programme with respect to the number of sampling stations and sampling
frequency. From 1998 zooplankton was given even lower priority in open
waters, but is still included in the monitoring of many coastal waters.

In general the diversity of the zooplankton community decreases with
salinity from the North Sea to the Baltic Sea and from the open sea to
the low salinity estuaries. However, diversity is high in the Kattegat
and the Belt Sea due to the influence of both brackish and Atlantic species
as a result of the inflow of water from the Baltic Proper and the Skagerrak-North
Sea, respectively. The relative importance of protozooplankton, rotiferans,
meroplankton and cladocerans is highest in the estuaries and decreases
gradually towards the open sea. Conversely, the importance of copepods
increases towards the sea.

During the period 1983-96 the winter biomass of mesozooplankton was
generally low (2-5 mg C m-3) in the open waters, increasing
to a seasonal summer peak of about 30-40 mg C m-3. Interregional
differences were minor, also with respect to species composition. An analysis
of copepod biomass data revealed a significant but only slight reduction
in the mesozooplankton biomass from 1989 to 1997 at two out of three Kattegat
stations (Ærtebjerg et al.
1998).

In the estuaries the composition and seasonal succession of the zooplankton
community is much more variable than in the open sea. Both seasonal successions
and time trends differ markedly among the estuaries monitored. In the
Limfjord the mesozooplankton biomass has increased significantly since
1985, while the biomass of protozooplankton has decreased, though not
significantly. In Ringkøbing Fjord the biomass of mesozooplankton
has decreased significantly since 1989 while the protozooplankton biomass
has increased, though not significantly. In Roskilde Fjord no time trend
was observed in the biomass of either meso- or protozooplankton. The lack
of general inter-fjord time trends is probably due to the effect of local
conditions.

Eutrophication is believed to cause an increase in the relative importance
of gelatinous zooplankton vs. crustacean zooplankton. Throughout the monitoring
period several estuaries like Roskilde Fjord, Kertinge Nor and Limfjorden
have suffered from occasional blooms of gelatinous zooplankton such as
the jellyfish Aurelia aurita (Frederiksborg
Amt & Roskilde Amt 2002, Fyns
Amt 2002). The jellyfish graze on the mesozooplankton, which in turn
leads to lowered grazing on the phytoplankton. In Kertinge Nor it has
been shown that high abundance of jellyfish reduces the importance of
zooplankton as grazers on phytoplankton to almost negligible levels (Petersen
et al. 1999). Moreover, jellyfish are suspected to affect the recruitment
of fish negatively, both by grazing on fish eggs and larvae and by affecting
the feeding conditions for fish larvae and planktivorous fish by causing
a decrease in the abundance of prey organisms such as copepods. Jellyfish
themselves are in a sense a trophic dead end. Energy and organic matter
that could otherwise be channelled into harvestable organisms is turned
into non-utilisable jelly.

THE CLADOCERAN PENILIA AVIROSTRIS:
AN ADDITION TO THE DANISH FAUNACladocerans
have their main distribution in freshwater habitats and the number of
species in Danish marine water is limited. Usually they constitute only
a minor fraction of the mesozooplankton biomass, but due to parthenogenesis
they can increase rapidly in numbers when growth conditions are favourable
and occasionally they become quantitatively important grazers. The two
dominating genera Evadne and Podon are believed to graze
mainly on large phytoplankton cells and on protozooplankton (Egloff
et al. 1997).

In the autumn 2001 a new cladoceran species Penilia avirostris
was observed in plankton samples from Århus Bight and Kattegat (Ærtebjerg
et al. 2002). This species has its main distribution in tropical and
subtropical seas, where it at times dominates the mesozooplankton biomass.
It feeds mostly on nanoplankton (2-20 µm) (Turner
et al. 1988) and thus plays a different role in the pelagic food web
than the other marine cladoceran species. Due to this P. avirostris may
be an important link between bacterioplankton and higher consumers because
of its predation on bacterivorous flagellates. P. avirostris
was reported in the North Sea as early as 1948 and since 1999 it has been
a steady component of the zooplankton community in the southern and eastern
parts of the North Sea, typically in September/October. Since the first
record in the North Sea it has spread progressively northward. For example
it was found around Helgoland (Germany) in the early 1990s and was recorded
in Koster Fjorden (Sweden) in 1997. In 2001 P. avirostris was
observed in relatively low abundances of about 100 individuals m-3 in
the Kattegat region, but in August-September 2002 it was among the dominating
mesozoopankton species in the Sound with abundances up to 4000 individuals
m-3 (Per Juel Hansen,
pers comm). P. avirostris has the capacity to quickly build dense
populations and significantly influence the food web structure and the
fate of the primary production. Therefore it is important to follow closely
the occurrence and development of this new, invasive species which is
probably well established in Danish waters.